Several naturally occurring, antibiotically active peptides with useful therapeutic activity against pathogenic bacteria and other classes of microorganisms have recently been identified and isolated from insects, frogs and higher animals. These include cecropins, attacins, magainins, sarcotoxin, sapecin, bactenecins, alamethicins, defensins and PGLa.
Other naturally occuring peptides from microorganisms, from insects and from higher animals are generally known as toxins because they lyse red blood cells as well as other eukaryotic cells. These toxins include different hemolysins such as streptolysins, melittin, barbatolysin, paradaxins and delta hemolysin. It is known but it is not widely recognized that some toxins like melittin will also kill bacteria. Therefore, for purposes of this description they will be described as antibiotically active peptides.
The invention described and claimed in the above identified related application is based on the unexpected discovery that novel antibiotic molecules can be constructed by joining together at least two amino acid sequences from different antibiotic peptides. One advantage of such hybrid molecules is that they may be constructed to be shorter and therefore easier to synthesize than the natural peptides from which they are derived.
The hybrid peptides, in addition to their antibiotic activity, appear to have other features in common. For example they all contain about 20 to 40 amino acids and often they are more effective if their C-terminals are amidated or blocked in other ways. They are therefore potential candidates for commercial preparation by solid phase synthesis. Additionally, they all appear to contain certain sequences of amino acids which impart specific conformations, i.e. secondary structural characteristics to portions of the molecule. Often the N-terminal region is hydrophilic and basic, and the C-terminal region is hydrophobic. Some portions of the molecule have a tendency towards helicity, others do not. Some molecules contain relatively long sequences which are flexible, thus forming hinge regions in the molecule. Often the helical portions are amphipathic, i.e., they are characterized by a hydrophilic and a hydrophobic surface.
These antibiotics appear to function by forming ion channels in the cell membrane of the bacteria, or other organism which then leads to rupture of the membrane and then rupturing of the cell. They have been characterized as lytic or channel forming antibiotics. Binding of the peptides to the membrane results in the formation of channels which permit entry of ions through the channels into the cellular fluid. This increases the osmotic pressure of the cell and causes more fluid to enter it. The increase in internal pressure causes the cell to burst. The differing secondary characteristics of the various portions of the antibiotic peptides appear to be associated with their mode of action in penetrating the cell membrane and lysing the cell.
A very important current medical problem is to find antibiotics with enhanced potency against human pathogens, especially those for which no suitable antibiotic is now available or to which resistant organisms have emerged. One response to the emergence of organisms resistant to antibiotics has been to prepare synthetic derivatives of the antibiotics, but this approach has been limited by the availability of functional groups on the parent molecule that can be utilized as foci for preparing derivatives.
It would be useful to have available a pool of antibiotics of comparatively simple structure which could be synthesized with relative ease, and which at the same time would be susceptible to structural variations for the purpose of producing analogs useful against specific organisms for which no non-toxic antibiotic is presently available, or for improving activity against other organisms for which the presently available antibiotics are toxic to the host. Such compounds should also have sufficient in vivo stability to resist degradation by mammalian enzymes.
It has been discovered, as described in the above identified patent application, that naturally occurring peptides such as those mentioned above, and others like them, constitute such a pool. In these compounds all of the amino acid residues are in the L-, or natural form.